Cellulose acetate film and method for producing cellulose acetate film

ABSTRACT

An object of the present disclosure is to provide a cellulose acetate film having excellent bending properties and high transparency. The subject cellulose acetate film contains cellulose acetate having a cellulose triacetate I crystal structure, the cellulose acetate film having a light transmittance of 70% or higher at 660 nm.

TECHNICAL FIELD

The present invention relates to a cellulose acetate film and a methodfor producing a cellulose acetate film.

BACKGROUND ART

In recent years, cellulose has been attracting attention as anaturally-occurring, environmentally-friendly biomass material. As themost abundant polysaccharide on Earth, cellulose can be found in thecell walls of plants, excretions of microorganisms, and the mantle ofthe ascidians. Cellulose has biodegradability, high crystallinity, andexcellent stability and safety. Therefore, it is expected to be appliedto a variety of fields. Among these, cellulose nanofibers, which areobtained by subjecting a cellulose material such as wood pulp tomechanical fibrillation treatment and refining the material to fibrilsor microfibrils, have characteristics such as high strength and highheat resistance, and are being actively studied as additives for resinsand as various functional base materials.

Proposed applications of cellulose nanofibers include a filter medium inthe shape of a sheet, a battery member such as a separator, and atransparent base material. For example, Patent Document 1 describes amethod for producing a sheet-like non-woven fabric by subjecting acellulose nanofiber dispersion to a papermaking process, the resultingproduct being used as a filter medium or battery member. The documentalso describes that: heat resistance and solvent resistance as a sheetcan be exhibited due to the degree of polymerization of the cellulosefibers being greater than or equal to 500; and the strength of the sheetcan be improved by adding a water-soluble polymer such as awater-soluble polysaccharide or a water-soluble polysaccharidederivative. However, although strength of the sheet is improved whenonly cellulose having a high degree of polymerization is used, the sheetbecomes hard and brittle and has worse bending properties. As a result,handleability, moldability, and processability are significantlyreduced, leading to problems in actual use. Furthermore, in a system inwhich a water-soluble polysaccharide or a water-soluble polysaccharidederivative is added, the number of work processes increases; meanwhile,the added water-soluble polymer is removed together with the filtrateduring the papermaking process, resulting in a low yield, and making itdifficult to benefit from the addition of water-soluble polymer.

Patent Document 2 describes a method for producing a microfibrouscellulose sheet, and that the fiber length of the microfibrous celluloseis preferably from 1 to 1000 μm while the aspect ratio thereof ispreferably from 100 to 30000. However, the resulting sheet of cellulosefibers in these ranges tends to be cloudy.

Patent Document 3 describes a cellulose nanofiber sheet containingchemically modified cellulose nanofibers and non-chemically modifiedcellulose nanofibers. The document also describes a cellulose nanofibersheet having a light transmittance of 70% or higher at 660 nm and anelongation of 2% or higher after being conditioned at 23° C. in ahumidity of 50%. In particular, one example disclosed a cellulosenanofiber sheet having an elongation of 6.8%. However, bendingproperties such as folding endurance are, in general, decided byelongation (elongation at break), and while higher elongation tends tolead to better bending properties, a sheet-like material (or film-likematerial) having even better bending properties is desirable from theperspective of handleability, moldability, and processability of thesheet.

CITATION LIST Patent Document

-   Patent Document 1: JP 2012-036529 A-   Patent Document 2: WO 2010/073678 A1-   Patent Document 3: JP 2016-211116 A

Non-Patent Literature

-   Non-Patent Literature 1: Takashi Nishino et al., Elastic modulus of    the crystalline regions of cellulose triesters, Journal of Polymer    Science Part B: Polymer Physics, March 1995, pp 611-618-   Non-Patent Literature 2: Junji Sugiyama et al., Electron diffraction    study on the two crystalline phases occurring in native cellulose    from an algal cell wall, Macromolecules, 1991, 24 (14), pp 4168-4175-   Non-Patent Literature 3: E. Roche et al., Three-Dimensional    Crystalline Structure of Cellulose Triacetate II, Macromolecules,    1978, 11 (1), pp 86-94-   Non-Patent Literature 4: Stipanovic A J et al., Molecular and    crystal structure of cellulose triacetate I: A parallel chain    structure, Polymer, 1978, 19 (1), pp 3-8.-   Non-Patent Literature 5: Masahisa Wada et al., X-ray diffraction    study of the thermal expansion behavior of cellulose triacetate I,    Journal of Polymer Science Part B: Polymer Physics, Jan. 21, 2009,    pp 517-523-   Non-Patent Literature 6: Takanori Kobayashi et al., Investigation of    the structure and interaction of cellulose triacetate I crystal    using ab initio calculations, Carbohydrate Research, Mar. 31, 2014,    Volume 388, pp 61-66-   Non-Patent Literature 7: Pawel Sikorski et al., Crystal Structure of    Cellulose Triacetate I, Macromolecules, 2004, 37 (12), pp 4547-4553

SUMMARY OF INVENTION Technical Problem

As described above, it has been difficult to obtain a cellulose acetatefilm having excellent bending properties and transparency fromnaturally-derived or chemically-modified cellulose. In particular, ithas been difficult to obtain a cellulose acetate film having both alight transmittance of 70% or higher at 660 nm and an elongation of 7%or higher.

An object of the present disclosure is to provide a cellulose acetatefilm having excellent bending properties and high transparency.

Solution to Problem

The present disclosure firstly relates to a cellulose acetate filmcontaining cellulose acetate having a cellulose triacetate I crystalstructure, the cellulose acetate film having a light transmittance of70% or higher at 660 nm.

The cellulose acetate film may have an elongation of 7% or higher whenconditioned at 23° C. and 50% relative humidity.

A temperature, at which a weight loss of the cellulose acetate filmrelative to weight at 100° C. reaches 5%, may be 200° C. or higher whenthe cellulose acetate film is heated at a heating rate of 10° C./minunder a nitrogen atmosphere.

A temperature at which the weight loss of the cellulose acetate filmreaches 5% may be 250° C. or higher.

A combined sulfuric acid content in the cellulose acetate of thecellulose acetate film may be from 20 ppm to 500 ppm.

The present disclosure secondly relates to a method for producing acellulose acetate film, the method including: acetylating raw materialcellulose by reacting the raw material cellulose with acetic anhydridein a solvent containing a poor solvent for cellulose acetate and aceticacid; diluting the cellulose acetate obtained by the acetylation with adispersion medium to prepare a dispersion; fibrillating the celluloseacetate in the dispersion; removing non-fibrillated fibers from thefibrillated cellulose acetate; dialyzing cellulose acetate from whichthe non-fibrillated fibers have been removed against water; and dryingthe dialyzed cellulose acetate to form a film.

Advantageous Effects of Invention

The present disclosure can provide a cellulose acetate film havingexcellent bending properties and high transparency.

DESCRIPTION OF EMBODIMENTS

Cellulose Acetate Film

The cellulose acetate film according to an embodiment of the presentdisclosure contains cellulose acetate having a cellulose triacetate Icrystal structure, the cellulose acetate film having a lighttransmittance of 70% or higher at 660 nm. Note that in the presentdisclosure, the terms “sheet” and “film” both indicate the shape of athin film and do not refer to objects that are specificallydistinguished by thickness.

Light Transmittance

The cellulose acetate film according to an embodiment of the presentdisclosure has a light transmittance of 70% or higher at 660 nm. Thelight transmittance is preferably not less than 80%, more preferably notless than 85%. When light transmittance at 660 nm is less than 70%,visible light cannot be efficiently transmitted, leading to inferiortransparency. In addition, while the light transmittance is preferablyhigher, with 100% being the most preferable, it may be 95% or below whentaking into consideration that reflection due to the difference inrefractive index at the interface between the air and the film isinevitable.

In addition, the cellulose acetate film according to an embodiment ofthe present disclosure can not be easily colored even by heating and canmaintain excellent transparency. In other words, the cellulose acetatefilm according to an embodiment of the present disclosure has excellentheat resistance. The cellulose acetate film according to an embodimentof the present disclosure preferably has a light transmittance at 450 nmof 70% or higher, more preferably 80% or higher, even after beingsubjected to heating for 3 hours at 100° C.

The light transmittance at 450 nm and 660 nm described above may both bemeasured by a spectrophotometer.

Elongation

The cellulose acetate film according to an embodiment of the presentdisclosure preferably has an elongation of 7% or higher, more preferably8% or higher, when conditioned at 23° C. and 50% relative humidity. Whenthe elongation is 7% or higher, the cellulose acetate film according toan embodiment of the present disclosure has higher folding endurance andexcellent bending properties. Furthermore, the elongation may be 16% andbelow from the perspective that elastic modulus tends to be negativelyaffected when elongation is too high.

The elongation may be measured by conditioning a cellulose acetate filmat 23° C. and 50% relative humidity for approximately 12 to 20 hours,and then performing a tensile test at, for example, a tensile width of10 mm, a span of 10 mm, and a speed of 5 mm/min.

Temperature at which Weight Loss Reaches 5%

The cellulose acetate film according to an embodiment of the presentdisclosure has a temperature, at which a weight loss relative to weightat 100° C. reaches 5%, of preferably 200° C. or higher, more preferably220° C. or higher, and even more preferably 250° C. or higher, when thecellulose acetate film is heated at a heating rate of 10° C./min under anitrogen atmosphere. This is preferable because a cellulose acetate filmhas better thermal stability. Meanwhile, a temperature at which theweight loss reaches 5% may be 350° C. or lower.

The weight loss can be measured using a thermobalance (TG-DTA2000-Savailable from MAC Science Co., Ltd.). Specifically, the celluloseacetate is heated at a heating rate of 10° C./min under a nitrogenatmosphere, and the weight change (relationship between the temperatureand the weight) is measured. Then, the weight loss (%) at eachtemperature relative to the weight of the cellulose acetate at 100° C.is calculated.

Cellulose Triacetate I Crystal Structure

The cellulose acetate film according to an embodiment of the presentdisclosure contains cellulose acetate having a cellulose triacetate Icrystal structure.

The fact that the cellulose acetate has a cellulose triacetate I(hereinafter also referred to as “CTA I”) crystal structure can beidentified by the appearance of typical peaks at two positions around2θ=7.6° (from 7.2 to 8.0°) and 2θ=15.9° (from 15.5 to 16.3°) in adiffraction profile obtained from an X-ray diffraction photograph usingCuKα (λ=1.542184 Å).

Similarly, the fact that cellulose acetate has a cellulose triacetate IIcrystal structure (hereinafter also referred to as “CTA II”) can beidentified by the appearance of typical peaks at three positions around2θ=7.9 to 8.9°, 2θ=9.9 to 10.9°, and 2θ=12.6 to 13.6°.

The cellulose acetate film according to an embodiment of the presentdisclosure contains cellulose acetate having a cellulose triacetate Icrystal structure and thus can have a small density and excellentstrength.

The crystal structures of cellulose and cellulose acetate are nowdescribed. As the crystal structures of cellulose, there are a celluloseI crystal structure and a cellulose II crystal structure (Non-PatentLiteratures 1 and 3). It is known that as the crystal structures ofcellulose acetate obtained by modifying cellulose with acetyl groups,there are a cellulose triacetate I crystal structure (CTA I) and acellulose triacetate II crystal structure (CTA II). (Non-PatentLiteratures 1, and 3 to 7). The cellulose triacetate I crystal structureis considered to be a parallel-chain structure similar to that of thecellulose I crystal structure (Non-Patent Literature 4), and thecellulose triacetate II crystal structure is considered to be anantiparallel-chain structure (Non-Patent Literature 3). Further, it isconsidered that once a cellulose triacetate I crystal structure isconverted to a cellulose triacetate II crystal structure, conversion toa cellulose triacetate I crystal structure does not occur (Non-PatentLiterature 3).

The small specific gravity, high strength, and low linear thermalexpansion coefficient of a cellulose nanofiber obtained from naturalcellulose are considered to result from the fact that the cellulosenanofiber has a cellulose I crystal structure (cellulose I, and moreprecisely, cellulose I is a mixture of cellulose Iα and cellulose Iβ.(Non-Patent Literature 2)) in which all the cellulose molecular chainsare oriented in the same direction to form a parallel-chain structure,and further result from the fact that the cellulose nanofiber has amicrofibril fiber structure, containing a cellulose I crystal structure,in which about 36 cellulose molecular chains are assembled and arrangedin parallel.

Average Degree of Substitution

The cellulose acetate of the cellulose acetate film according to anembodiment of the present disclosure preferably has an average degree ofsubstitution (in other words, an acetyl substitution degree) from 2.0 to3.0, more preferably from 2.1 to 2.9, and even more preferably from 2.2to 2.8. When the acetyl substitution degree is within these ranges, thecellulose acetate has high hydrophobicity on the molecular surface, andthe cellulose acetate film has excellent bending properties.

The average degree of substitution of the cellulose acetate can bemeasured by a known titration method in which cellulose acetate isdissolved in water and the average degree of substitution of thecellulose acetate is determined. For example, the following method canbe used. A combined acetic acid of cellulose acetate is determinedaccording to a method for measuring combined acetic acid specified inASTM:D-817-91 (test methods for cellulose acetate etc.) and converted toaverage substitution degree by the following formula. This is the mostcommon method for determining the average degree of substitution of thecellulose acetate.Average degree of substitution (DS)=162.14×the combined acetic acid(%)/{6005.2−42.037×the combined acetic acid (%)}

First, 1.9 g of dried cellulose acetate (sample) is precisely weighedand dissolved in 150 mL of a mixed solution of acetone anddimethylsulfoxide (volume ratio 4:1), and then 30 mL of a 1 N aqueoussodium hydroxide solution was added to saponify the cellulose acetate at25° C. for 2 hours. Phenolphthalein is added as an indicator, and theexcess sodium hydroxide is titrated with 1N-sulfuric acid (concentrationfactor: F). Further, a blank test is performed in the same manner, andthe combined acetic acid of the sample is calculated by the followingformula:Combined acetic acid (%)={6.5×(B−A)×F}/W

where A is a titration volume (mL) of 1N-sulfuric acid used for thesample, B is a titration volume (mL) of 1N-sulfuric acid used in theblank test, F is a concentration factor of 1N-sulfuric acid, and W is aweight of the sample.

Combined Sulfuric Acid Content

The cellulose acetate of the cellulose acetate film according to anembodiment of the present disclosure has a combined sulfuric acidcontent of preferably 500 ppm or less, more preferably 400 ppm or less,even more preferably 350 ppm or less, and most preferably 300 ppm orless. This is preferable because a cellulose acetate film has betterthermal stability. In addition, the cellulose acetate may have acombined sulfuric acid content of 20 ppm or greater and 50 ppm orgreater. This is because sulfuric acid is preferably used as a catalystfor the acetylation in the process of producing the cellulose acetatefilm, and when an effective amount of sulfuric acid as the catalyst forthe acetylation is used, the combined sulfuric acid content in theresulting cellulose acetate is 20 ppm or greater.

The combined sulfuric acid content can be determined by the followingmethod. A dried cellulose acetate is burned in an electric oven at 1300°C., and the sublimated sulfurous acid gas is trapped in a 10% hydrogenperoxide water and titrated with a normal aqueous solution of sodiumhydroxide to measure the content in terms of SO₄ ²⁻. The measured valueis expressed in ppm as a sulfate content in 1 g of the cellulose esterin the absolute dry state.

Viscosity-Average Degree of Polymerization

The viscosity-average degree of polymerization of the cellulose acetateof the cellulose acetate film according to an embodiment of the presentdisclosure is preferably from 50 to 2500, more preferably from 400 to2000, and even more preferably from 1000 to 1500. When theviscosity-average degree of polymerization is less than 50, thecellulose acetate tends to have poor strength. When theviscosity-average degree of polymerization exceeds 2500, it is difficultto perform defibrillation to allow the cellulose acetate fibers to havea number-average fiber diameter of 2 nm or greater and 400 nm orsmaller.

The viscosity-average degree of polymerization (DP) can be determined bya method described by Kamide et al. in Polym J., 11, 523-538 (1979).

Cellulose acetate is dissolved in dimethylacetamide (DMAc) and asolution having a concentration of 0.002 g/mL is prepared. Then, thespecific viscosity (η_(rel), unit: mL/g) of this solution is determinedat 25° C. by an ordinary method using an Ostwald viscometer. Morespecifically, the Ostwald viscometer used has efflux time in a blanktest of 90 seconds to 210 seconds, the temperature of the solution to bemeasured is regulated in a thermostatic bath at 25±0.2° C. for 120minutes or more, 10 mL of the solution is introduced into the Ostwaldviscometer using a transfer pipette, and the efflux time of the solutionis measured twice or more to determine an average as a measurementresult. The measurement result is divided by the efflux time of a blankmeasured in the same manner to determine a specific viscosity. Thenatural logarithm of the thus determined specific viscosity (naturallogarithmic of the specific viscosity) is divided by the concentration(unit: g/mL) to approximately determine a value of intrinsic viscosity([η], unit: mL/g).η_(rel) =T/T ₀[η]=(ln η_(rel))/C

where T represents the efflux time (in seconds) of the measurementsample, T₀ represents the efflux time (in seconds) of the solvent alone,and C represents the concentration (g/mL).

The viscosity-average molecular weight can be determined by thefollowing equation:Viscosity-average molecular weight=([η]/K _(m))^(1/α)

where K_(m) and α are constants. In the case of cellulose triacetate,K_(m) is 0.0264 and α is 0.750.

The viscosity-average degree of polymerization can be determined by thefollowing equation:Viscosity-average degree of polymerization=viscosity-average molecularweight/(162.14+42.037×average degree of substitution (DS))

The cellulose acetate of the cellulose acetate film according to anembodiment of the present disclosure may be fibrous cellulose acetate,in other words, cellulose acetate fiber.

Number-Average Fiber Diameter

The number-average fiber diameter of the cellulose acetate fiber may befrom 2 nm to 400 nm. The number-average fiber diameter is preferably 4nm or greater and 300 nm or smaller, and more preferably 6 nm or greaterand 100 nm or smaller.

Here, the number-average fiber diameter of the cellulose acetate fiberis a value calculated from the fiber diameter (n≥6) measured based on anelectron micrograph.

Application

The cellulose acetate film according to an embodiment of the presentdisclosure can be used in, for example, filter media, battery memberssuch as separators, and transparent base materials.

Production of Cellulose Acetate Film

The cellulose acetate film according to an embodiment of the presentdisclosure can be produced by a method including the following steps:acetylating raw material cellulose by reacting the raw materialcellulose with acetic anhydride in a solvent containing a poor solventfor cellulose acetate and acetic acid; diluting the cellulose acetateobtained by the acetylation with a dispersion medium to prepare adispersion; fibrillating the cellulose acetate in the dispersion;removing non-fibrillated fibers from the fibrillated cellulose acetate;dialyzing cellulose acetate from which the non-fibrillated fibers havebeen removed against water; and drying the dialyzed cellulose acetate toform a film.

Raw Material Cellulose

A fibrous material such as wood pulp or cotton linters, andparticularly, a fibrous material having a cellulose I crystal structure,can be used as the raw material cellulose. These raw material cellulosesmay be used alone or in combination of two or more. In this way, thecellulose acetate film according to an embodiment of the presentdisclosure effectively utilizes cellulose material, which is a naturallyoccurring biomass material.

Cotton linters will be described. Linter pulp has a high cellulosepurity and contain fewer colored components. Therefore, linter pulp ispreferred because the resulting cellulose acetate film has highertransparency.

Next, wood pulp will be described. Wood pulp is preferred because woodpulp can be stably supplied as a raw material and has a cost advantageover cotton linters.

Examples of the wood pulp include softwood pulp and hardwood pulp, andspecific examples of the softwood pulp and the hardwood pulp includesoftwood bleached kraft pulp, hardwood bleached kraft pulp, softwoodprehydrolyzed kraft pulp, hardwood prehydrolyzed kraft pulp, hardwoodsulfite pulp, and softwood sulfite pulp. As will be described later,wood pulp can be disintegrated into fluff and used as disintegratedpulp. The disintegration can be performed using, for example, a discrefiner.

The α-cellulose content of the raw material cellulose is preferably 90wt. % and above. This is so that the insoluble residue is reduced, andthe resulting cellulose acetate film has higher transparency.

The α-cellulose content can be determined in the following manner. Pulphaving a known weight is continuously subjected to extraction at 25° C.using a 17.5% aqueous sodium hydroxide solution and a 9.45% aqueoussodium hydroxide solution, and then a soluble fraction in the extractionsolution is oxidized with potassium dichromate. The weight ofβ,γ-cellulose is determined from the volume of potassium dichromate usedfor oxidization. A value obtained by subtracting the weight ofβ,γ-cellulose from the initial weight of the pulp is defined as theweight of insoluble fraction of the pulp, that is, the weight ofα-cellulose (TAPPI T203). The ratio of the weight of insoluble fractionof the pulp to the initial weight of the pulp is defined as theα-cellulose content (wt. %).

Disintegration

The method for producing a cellulose acetate film according to anembodiment of the present disclosure may include a step ofdisintegrating raw material cellulose (disintegration step). This makesit possible to uniformly perform an acetylation reaction in a shorttime. The disintegration step is particularly effective when wood pulpor the like is supplied in the form of sheets.

In the disintegration step, raw material cellulose is disintegrated by awet disintegration or a dry disintegration. The wet disintegration is amethod in which water or water vapor is added to wood pulp such as pulpsheets to disintegrate the wood pulp. Examples of the wet disintegrationinclude: a method in which activation by water vapor and high-shearstirring in a reactor are performed; and a method in whichdisintegration is performed in a dilute aqueous acetic acid solution toobtain a slurry, and then the slurry is repeatedly subjected to liquidremoval and acetic acid substitution, that is, so-called slurrypretreatment is performed. The dry disintegration is a method in whichwood pulp, such as pulp sheets, is directly disintegrated in a drystate. Examples of the dry disintegration include: a method in whichpulp is roughly disintegrated by a disc refiner having pyramid teeth andthen finely disintegrated by a disc refiner having linear teeth; and amethod in which a turbo mill is used which includes a cylindrical outercase having a liner attached to its inner wall, a plurality of discsthat rotate at a high speed about the center line of the outer case, anda large number of blades radially attached around the center line insuch a manner that each of the blades is located between the discs, andan object to be disintegrated is supplied into the outer case anddisintegrated by three kinds of impact actions caused by hitting by theblades, collision with the liner, and high-frequency pressureoscillation generated by the action of the high-speed rotating discs,the blades, and the liner.

In the method for producing a cellulose acetate film according to anembodiment of the present disclosure, any one of these disintegrationmethods may be appropriately used. The wet disintegration isparticularly preferred because the acetylation reaction can be completedin a short time and cellulose acetate having a high degree ofpolymerization can be obtained.

Pretreatment

The method for producing a cellulose acetate film according to anembodiment of the present disclosure preferably includes a pretreatmentstep in which disintegrated or non-disintegrated raw material celluloseis brought into contact with water, acetic acid, or water and aceticacid. The raw material cellulose may be brought into contact with waterand acetic acid or may be brought into contact with only acetic acidwithout water. Here, acetic acid with a concentration from 1 to 100 wt.% can be used. Acetic acid may be an aqueous solution. Water, aceticacid, or water and acetic acid can be brought into contact with rawmaterial cellulose by, for example, adding preferably from 10 to 8000parts by weight of water, acetic acid, or water and acetic acid per 100parts by weight of raw material cellulose.

Examples of a method for bringing raw material cellulose into contactwith acetic acid include a method in which raw material cellulose isbrought into direct contact with acetic acid and a method in which rawmaterial cellulose is brought into contact with water to prepare awater-containing wet cake and acetic acid is added to the wet cake.

Examples of the method in which raw material cellulose is brought intodirect contact with acetic acid include: a method in which acetic acidor acetic acid containing from 1 to 10 wt. % of sulfuric acid(sulfur-containing acetic acid) is added in one step; and a method inwhich acetic acid or sulfur-containing acetic acid is added in two ormore separate steps, such as a method in which acetic acid is firstadded and then sulfur-containing acetic acid is added after a certainperiod of time, or a method in which sulfur-containing acetic acid isfirst added and then acetic acid is added after a certain period oftime. More specifically, acetic acid and/or sulfur-containing aceticacid may be added by spraying it or them onto raw material cellulose andmixing the raw material cellulose.

The pretreatment can then be performed, for example, by adding aceticacid and/or the sulfur-containing acetic acid to the raw materialcellulose and then allowing the mixture to stand at 17 to 40° C. for 0.2to 48 hours or sealing and stirring the mixture at 17 to 40° C. for 0.1to 24 hours.

The case where a wet cake of raw material cellulose is prepared beforethe raw material cellulose is brought into contact with acetic acid willbe described. Here, raw material cellulose in the form of a wet cake issimply referred to as a wet cake. The wet cake can be produced by addingwater to raw material cellulose, stirring the mixture, and separatingthe water by filtration. The raw material cellulose can be pretreated byrepeating the operation of adding acetic acid to the wet cake, stirringthe mixture, and separating the acetic acid by filtration several times,for example, about three times. The solid content concentration of thewet cake just after separating water or acetic acid by filtration ispreferably from 5 to 50 wt. %.

When a wet cake of raw material cellulose is prepared, the raw materialcellulose is preferably softwood bleached kraft pulp or softwoodbleached sulfite pulp. This is because cellulose acetate having arelatively high degree of polymerization and excellent strength can beeasily obtained.

Here, the solid content concentration of the wet cake can be determinedin the following manner. About 10 g of part of a wet cake (sample) isweighed on an aluminum tray (W2), dried in a vacuum dryer at 60° C. for3 hours, cooled in a desiccator to room temperature, and weighed (W3) todetermine the solid content concentration of the sample according to thefollowing formula:Solid content concentration (%)=(W3−W1)/(W2−W1)×100

where W1 is a weight (g) of the aluminum tray, W2 is a weight (g) of thealuminum tray containing a sample before drying, and W3 is a weight (g)of the aluminum tray containing a dried sample.

When a wet cake of raw material cellulose is brought into contact withacetic acid, acetylation can be performed at a relatively lowtemperature in a relatively short time in an acetylation step that willbe described later. This allows for easy control of temperatureconditions and time conditions and easy handling of the raw materialcellulose, which in turn increases the production efficiency ofcellulose acetate film.

Acetylation

The following is the detailed description of the step of acetylating rawmaterial cellulose by reacting it with acetic anhydride in a solventcontaining a poor solvent for cellulose acetate and acetic acid(acetylation step). The raw material cellulose includes raw materialcellulose that has been subjected to the disintegration step and thepretreatment step as well as raw material cellulose that has not beensubjected to these steps.

More specifically, acetylation can be initiated by, for example, i)adding a poor solvent for cellulose acetate, acetic acid, aceticanhydride, and sulfuric acid to raw material cellulose. The order ofaddition may be different from the order above. Alternatively,acetylation may be initiated by ii) adding raw material cellulose to amixture of a poor solvent for cellulose acetate, acetic acid, aceticanhydride, and sulfuric acid or iii) adding a previously-preparedmixture, such as a mixture of acetic acid, a poor solvent for celluloseacetate, and acetic anhydride, and sulfuric acid to raw materialcellulose. The acetic acid used here is preferably one having aconcentration of 99 wt. % or greater. The sulfuric acid used here ispreferably one having a concentration of 98 wt. % or greater, that is,concentrated sulfuric acid.

By using a poor solvent for cellulose acetate, acetylation can beperformed without breaking the microfibril fiber structure of the rawmaterial cellulose. If the poor solvent is not used, produced celluloseacetate is dissolved in acetic acid used as a diluent in the acetylationreaction and the microfibril structure of the raw material cellulose isbroken.

Needless to say, the poor solvent for cellulose acetate does notdissolve or hardly dissolves cellulose acetate. In addition, the poorsolvent for cellulose acetate preferably well dissolves aceticanhydride. Examples of such a poor solvent for cellulose acetateinclude: aromatic hydrocarbons such as benzene, toluene, and xylene;aliphatic hydrocarbons such as cyclohexane and hexane; esters such asamyl acetate; and mixed solvents of two or more of them.

Among them, toluene and cyclohexane are preferred, and benzene is morepreferred because the number of steps for separating and collectingwaste liquid can be reduced or energy required for collection can bereduced.

The ratio between raw material cellulose, acetic acid, a poor solventfor cellulose acetate, and acetic anhydride used in the acetylation stepwill be described on for each case.

A case in which pretreatment is performed by bringing acetic acid intodirect contact with raw material cellulose is described. The amount of apoor solvent for cellulose acetate is preferably from 100 to 5000 partsby weight, more preferably from 1000 to 2000 parts by weight per 100parts by weight of raw material cellulose. The amount of acetic acid ispreferably 0 to 2000 parts by weight, more preferably from 50 to 1000parts by weight per 100 parts by weight of raw material cellulose. Theamount of acetic anhydride is preferably from 200 to 1000 parts byweight, more preferably from 300 to 700 parts by weight per 100 parts byweight of raw material cellulose. When sulfuric acid is used as acatalyst, the amount of sulfuric acid is preferably from 1 to 30 partsby weight, more preferably from 5 to 20 parts by weight per 100 parts byweight of raw material cellulose.

A case in which raw material cellulose is pretreated with water toprepare a wet cake of the raw material cellulose before the raw materialcellulose is brought into contact with acetic acid is described. Whenthe solid content concentration of the wet cake is from 5 to 50 wt. %,the amount of acetic acid is preferably from 100 to 4000 parts byweight, more preferably from 200 to 3000 parts by weight, even morepreferably from 1000 to 2000 parts by weight per 100 parts by weight ofthe wet cake. The amount of a poor solvent for cellulose acetate ispreferably from 5 to 2500 parts by weight, more preferably from 50 to1000 parts by weight per 100 parts by weight of the wet cake. The amountof acetic anhydride is preferably from 5 to 1000 parts by weight, morepreferably from 10 to 500 parts by weight, even more preferably from 15to 350 parts by weight per 100 parts by weight of the wet cake. Theamount of sulfuric acid is preferably from 0.05 to 15 parts by weight,more preferably from 5 to 10 parts by weight per 100 parts by weight ofthe wet cake.

The temperature in the reaction system during the acetylation step ispreferably 5 to 90° C., more preferably 10 to 75° C. In a case where thetemperature in the acetylation reaction system is too high,depolymerization of the raw material cellulose is likely to proceed,causing excessive reduction in the viscosity-average degree ofpolymerization and reduction in the strength of the produced celluloseacetate fibers. On the other hand, in a case where the temperature inthe acetylation reaction system is too low, the acetylation reactiondoes not proceed. As a result, the reaction requires an enormous amountof time, or conversion of cellulose to cellulose acetate cannot beperformed.

The temperature in the acetylation reaction system can be adjusted byexternally applying no heat to the inside and outside of the reactionsystem under stirring conditions, and/or heating or cooling the reactionsystem using a heating medium or a coolant under stirring conditionssuch that the reaction system is adjusted to an intermediatetemperature. Alternatively, the temperature in the reaction system maybe adjusted by heating or cooling acetic acid, a poor solvent forcellulose acetate, acetic anhydride, and sulfuric acid in advance.

The time taken for the acetylation reaction is preferably 0.5 to 20hours. Here, the time taken for the acetylation reaction refers to thetime from when raw material cellulose is brought into contact with asolvent, acetic anhydride, and a catalyst to start the reaction untilwhen a product (cellulose acetate) is separated from a reaction mixtureby filtration or the like. However, when chemically-modified pulp, suchas TEMPO oxidized pulp, is used as raw material cellulose, the timetaken for the acetylation reaction is preferably 0.5 to 60 hours.

In the early stage of the acetylation reaction, the reaction temperaturemay be 5° C. or lower such that the acetylation reaction is allowed toproceed while the depolymerization reaction is suppressed to reduce theamount of unreacted materials. In this case, the reaction temperatureshould be increased as slowly as possible, but from the viewpoint ofproductivity, the reaction temperature is preferably increased in 45minutes or less, more preferably 30 minutes or less.

The average degree of substitution can be adjusted by adjusting thetemperature or time of the acetylation reaction and the composition of areaction bath such as the amount of acetic anhydride or the amount ofsulfuric acid. For example, the average degree of substitution can beincreased by increasing the temperature, prolonging the time, increasingthe amount of sulfuric acid, or increasing the amount of aceticanhydride.

The acetylation reaction may be stopped by separating cellulose acetateas a solid from the reaction mixture of the acetylation and washing thesolid with a poor solvent for cellulose acetate, such as toluene, toremove acetic anhydride and sulfuric acid. The separation can beperformed by cooling the reaction mixture of the acetylation reactionand filtering the resulting solid. The filtration may be suctionfiltration.

Preparation of Dispersion

The following is the detailed description of the step of dilutingcellulose acetate obtained by the acetylation with a dispersion mediumto prepare a dispersion. The dispersion medium may be water, an organicsolvent, or an organic solvent containing water. At this time, theamount of cellulose acetate is preferably from 0.1 to 10 wt. %, morepreferably from 0.5 to 5.0 wt. %, with respect to the amount of water,an organic solvent, or an organic solvent containing water. In thefibrillation step that will be described later, when the solid contentconcentration is less than 0.1 wt. %, the amount of a liquid to betreated is too large, leading to reduced industrial productionefficiency. Meanwhile, when the solid content concentration is greaterthan 10 wt. %, the fibrillation step may not proceed due to, forexample, clogging in a fibrillation device.

Examples of the organic solvent include methanol, ethanol, 2-propanol,acetone, tetrahydrofuran, and methyl acetate. In addition, a mixture ofthese organic solvents and water can be also used.

When the acetylation reaction is stopped by washing with a poor solventfor cellulose acetate such as toluene, the cellulose acetate may bewashed in advance with, for example, ethanol, before being diluted witha dispersing medium.

Defibrillation

The following is the detailed description of the step of fibrillatingcellulose acetate in the dispersion (fibrillation step). In this way,the cellulose acetate can be refined into finer fibers. The fibrillationmay be performed using a homogenizer.

A device used for defibrillation is not particularly limited, but ispreferably one that can apply a strong shearing force, such as ahigh-speed rotation device, a colloid mill, a high-pressure device, aroll mill, and an ultrasonic device. Particularly, a wet high- orultrahigh-pressure homogenizer that can apply a pressure of 50 MPa orgreater and a strong shearing force to the dispersion liquid ispreferably used to efficiently perform fibrillation. The pressure ismore preferably 100 MPa or greater, even more preferably 140 MPa orgreater. Prior to defibrillation and dispersion with the use of ahigh-pressure homogenizer, if necessary, the cellulose acetate may besubjected to pretreatment using a known mixing, stirring,emulsification, or dispersing device such as a high-shear mixer.

Here, when the pressure is 50 MPa or greater, resulting celluloseacetate fibers can have a number-average fiber diameter of 400 nm orless, and when the pressure is 100 MPa or greater, the number-averagefiber diameter can be made smaller.

In the method for producing a cellulose acetate film according to anembodiment of the present disclosure, raw material cellulose isacetylated in a solvent containing a poor solvent for cellulose acetateand then fibrillated using a homogenizer. And thus, resulting celluloseacetate can maintain a microfibril fiber structure of natural cellulose.

Desulfation

In the method for producing a cellulose acetate film according to anembodiment of the present disclosure, when the cellulose acetatecontains sulfate groups, it is preferable to perform desulfation, thatis, removal of sulfate groups, after fibrillation of the celluloseacetate.

The sulfate groups can be removed by adjusting the pH of the dispersioncontaining cellulose acetate to 2 to 5 and keeping the dispersion at 15to 100° C. for 0.5 to 48 hours.

Removal of Non-Fibrillated Fibers

The following is the detailed description of the step of removingnon-fibrillated fibers from the fibrillated cellulose acetate. Byremoving the non-fibrillated fibers, impurities can be removed, and acellulose acetate film having excellent bending properties and hightransparency can be obtained. It is preferable to use a dispersion ofcellulose acetate that does not contain non-fibrillated fibers. Theremoval of the non-fibrillated fibers may be performed by centrifugingto separate into supernatant and precipitate, removing the precipitate,and keeping the supernatant.

Dialysis

The following is the detailed description of the step of dialyzing thecellulose acetate from which non-fibrillated fibers has been removedagainst water. The supernatant obtained by centrifuging may be dialyzedagainst water using a dialysis membrane. By performing dialysis,impurities can be removed, and a cellulose acetate film having excellentbending properties and high transparency can be obtained. Examples ofthe dialysis membrane that can be used include Cellulose Tubing 36/32(available from Viskase Companies, Inc.).

Film Formation

The following is the detailed description of the step of drying thedialyzed cellulose acetate to form a film. Cellulose acetate can beformed into a film by concentrating the dialyzed supernatant, placing itin a mold, and drying it. Drying is preferably performed in the presenceof a desiccant such as diphosphorus pentoxide.

Each aspect disclosed in the present specification can be combined withany other feature disclosed herein.

EXAMPLES

Hereinafter, the present disclosure will be specifically described withreference to examples, but the technical scope of the present disclosureis not limited by these examples.

Each physical property of Examples and Comparative Examples to bedescribed later were evaluated by the following methods.

Crystal Structure: X-Ray Diffraction

The crystal structure was examined by rendering a film sample (or sheetsample) into powder and subjecting the resulting powder to powder X-raydiffraction using an X-ray diffraction measurement device SmartLab,available from Rigaku Corporation, and a non-reflecting silicon plate.

Light Transmittance

The light transmittance (%) of a film sample (or sheet sample) at 660 nmwas measured using a U-4000 spectrophotometer (available from Hitachi,Ltd.). In addition, the light transmittance (%) at 450 nm after heatingfor 3 hours at 100° C. was measured in the same manner.

Elongation

A film sample (or sheet sample) was conditioned overnight in anenvironment of 23° C. and 50% relative humidity; then, the film sample(or sheet sample) was pulled at a width of 10 mm, a span of 10 mm, and aspeed of 5 mm/min using a small tabletop testing instrument EZ-LXAutograph (available from Shimadzu Corporation), and the elongation(maximum increase in length) (%) was measured.

Temperature at which Weight Loss Reaches 5%

The weight change by heating was measured using a thermobalance(TG-DTA2000-S available from MAC Science Co., Ltd.). Specifically, theweight change was examined at a heating rate of 10° C./min under anitrogen atmosphere. A temperature (° C.) at which a weight loss reaches5% is a temperature at which a 5% weight loss relative to weight at 100°C. was observed.

Combined Sulfuric Acid Content

The combined sulfuric acid content was measured as a content in terms ofSO₄ ²⁻ by burning a dried sample in an electric oven at 1300° C.,trapping sublimated sulfurous acid gas in a 10% hydrogen peroxide water,and titrating with a normal aqueous solution of sodium hydroxide. Themeasured value was expressed in ppm as a sulfate content in 1 g of thecellulose ester in the absolute dry state.

Average Degree of Substitution: Degree of Acetyl Substitution

The combined acetic acid was determined by a method for measuring thecombined acetic acid specified in ASTM:D-817-91 (Test method ofcellulose acetate and the like). First, 1.9 g of a dried sample wasprecisely weighed and dissolved in 150 mL of a mixed solvent of acetoneand dimethylsulfoxide (volume ratio 4:1); then, 30 mL of a 1 N aqueoussolution of sodium hydroxide was added to saponify the sample at 25° C.for 2 hours. Phenolphthalein was added as an indicator, and excesssodium hydroxide was titrated with 1 N-sulfuric acid (concentrationfactor: F). Further, a blank test was performed in the same manner, andthe combined acetic acid was calculated by the following formula:Combined acetic acid (%)=[6.5×(B−A)×F]/W

where A represents the titration volume (mL) of the 1 N sulfuric acidfor the sample, B represents the titration volume (mL) of the 1 Nsulfuric acid for the blank test, F represents the concentration factorof the 1 N sulfuric acid, and W represents the weight of the sample.

The calculated combined acetic acid was converted by the followingequation to determine the average degree of substitution:Average degree of substitution (DS)=162.14×the combined acetic acid(%)/{6005.2−42.037×the combined acetic acid (%)}Viscosity-Average Degree of Polymerization

A sample was dissolved in dimethylacetamide (DMAc) to prepare a solutionhaving a concentration of 0.002 g/mL. Then, the specific viscosity(η_(rel), unit: mL/g) of this solution at 25° C. was determined by anordinary method using an Ostwald viscometer. The natural logarithm ofthe specific viscosity was divided by the concentration (unit: g/mL) toapproximately determine a value of intrinsic viscosity ([η], unit:mL/g).η_(rel) =T/T ₀[η]=(ln η_(rel))/C

where T represents the efflux time (in seconds) of the measurementsample, T₀ represents the efflux time (in seconds) of the solvent alone,and C represents the concentration (g/mL).

The viscosity-average molecular weight was determined by the followingequation:Viscosity-average molecular weight=([η]/K _(m))^(1/α)

wherein K_(m)=0.0264 and α=0.750.

Example 1

Pretreatment

First, 40 parts by weight of powdered cellulose (PC, product name “KCFLOCK W-50GK”, available from Nippon Paper Industries Co., Ltd.) waspretreated by stirring in 2000 parts by weight of water at roomtemperature for 1 hour. Liquid was removed by suction filtration toobtain a wet cake (PC) having a solid content concentration of about 20wt. %. This wet cake was dispersed in 2000 parts by weight of glacialacetic acid, and the dispersion was stirred at room temperature for 10minutes. Liquid was removed by suction filtration, and a wet cake (PC)wetted with acetic acid was obtained. The solid content concentration ofthis acetic acid-wetted wet cake (PC) was about 35 wt. %. An operationof dispersing this wet cake (PC) wetted with acetic acid in glacialacetic acid again and removing liquid was performed twice. The solidcontent concentration of the acetic acid-wetted wet cake (PC) thusobtained was about 40 wt. %. The solid content concentration of the wetcake was measured by the method described above.

Acetylation

A mixture (mixed solvent) was prepared by mixing 648 parts by weight oftoluene as a poor solvent for cellulose acetate, 72 parts by weight ofacetic acid, 240 parts by weight of acetic anhydride, and 6 parts byweight of concentrated sulfuric acid. The temperature of this mixturewas adjusted to 25° C.; the wet cake (PC) was added to the mixture, andthe mixture was stirred at 25° C. for 3 Hour® s to form a reactionmixture. This reaction mixture was cooled to room temperature, theresulting solid was suction-filtered, and the solid was recovered. Anoperation of immediately washing this solid with 800 parts by weight oftoluene was repeated twice to remove acetic anhydride and sulfuric acidassociated with the solid, and thereby the acetylation reaction wasstopped.

Washing and Preparation of Dispersion

The solid, which was crude cellulose acetate, was further washed with800 parts by weight of ethanol twice and with 800 parts by weight ofdistilled water four times, resulting in a wet cellulose acetate. Thewet cellulose acetate was diluted with distilled water to form adispersion having a solid content of 1 wt. %.

Defibrillation

The obtained dispersion of cellulose acetate was pre-fibrillated with anExcel Auto Homogenizer (available from Nihonseiki Kaisha Ltd.) and thenfibrillated by processing twice using a straight nozzle (100 MPa) andthree times using a cross nozzle (140 MPa) of a high-pressurehomogenizer (product name L-AS, available from Yoshida Kikai Co., Ltd.).A 1 wt. % dispersion of the fibrillated product of cellulose acetate wasthus obtained.

Desulfation

Approximately 1.2 parts by weight of 0.5 M sulfuric acid was added to400 parts by weight of the dispersion of the fibrillated product ofcellulose acetate obtained, and the pH of the dispersion was adjusted to2.5. The dispersion after pH adjustment was held at 90° C. for 6 hours,and the sulfate groups attached to the fibrillated product of celluloseacetate were removed. 1000 parts by weight of distilled water was addedto the dispersion; then, approximately 150 parts of a 5% aqueoussolution of sodium bicarbonate was added to the dispersion, and the pHof the dispersion was adjusted to 6.

Removal of Non-Fibrillated Fibers

The dispersion adjusted to pH 6 was centrifuged at room temperature at arotation speed of 4000 rpm for 15 minutes using a tabletop centrifuge4000 (available from KUBOTA Corporation) and separated into supernatantand precipitate. The supernatant was subjected to the followingexperiments.

Dialysis

The supernatant was dialyzed against an excess amount of distilled waterfor 3 days. A Cellulose Tubing 36/32 from Viskase Companies, Inc. wasused as the dialysis membrane.

Film Formation

The dialyzed supernatant was concentrated three-fold. The resultingproduct was placed in a Teflon (registered trademark) mold and vacuumdried for 3 days in the presence of diphosphorus pentoxide to obtain acellulose acetate film having a thickness of 20 μm.

Measurement of Physical Properties of Film

The physical properties of the obtained film are shown in Table 1.According to the measurements of the film sample, the combined sulfuricacid content was 290 ppm, the average degree of substitution was 2.35,the viscosity-average degree of polymerization was 585, and thetemperature at which weight loss reached 5% was 284° C. Furthermore, afilm sample was rendered into powder, treated at 230° C. for 10 minutesin a nitrogen atmosphere, and subjected to powder X-ray diffraction;diffraction was observed at 2θ=7.9° and 15.8°, and it was determinedthat the film sample had a cellulose triacetate I crystal.

Production Example 1

(1) TEMPO (2,2,6,6-tetramethyl-1-piperidine-N-oxyl) Oxidation Treatmentof Cellulose

30 g of softwood kraft pulp was immersed in 600 g of water and dispersedin a mixer. 0.3 g of TEMPO dissolved in 200 g of water and 3 g of sodiumbromide was added to the dispersed pulp slurry, and the resultingmixture was further diluted with water to make the volume 1400 mL. Theinside of the system was maintained at 20° C., and an aqueous solutionof sodium hypochlorite was weighed and added dropwise so as to reach 10mmol per 1 g of cellulose. The pH began to drop from the start of thedropwise addition of the aqueous solution of sodium hypochlorite; a 0.5N aqueous solution of sodium hydroxide was added dropwise as needed tomaintain the pH of the system at 10. After 3 hours, 30 g of ethanol wasadded to stop the reaction. 0.5 N hydrochloric acid was added to thereaction system to reduce the pH to 2. The oxidized pulp was filteredand washed repeatedly with 0.01 N hydrochloric acid or water to obtainan oxidized cellulose.

(2) Additional Oxidation Treatment of Oxidized Cellulose

Water was added to the oxidized cellulose obtained in (1) above so as toform a suspension having a solid content concentration of 10% withrespect to a dry weight of 10 g of the oxidized cellulose, and 9 g ofsodium chlorite and 100 mL of 5 M acetic acid were added to thesuspension. The resulting product was reacted with stirring at roomtemperature for 48 hours and washed thoroughly with water to oxidize thealdehyde groups generated by the TEMPO oxidation treatment.

(3) Dispersion Treatment of Oxidized Cellulose After AdditionalOxidation Treatment

Water was added to 5 g of the oxidized cellulose obtained in (2) aboveto adjust the solid content to 1%, and a 1 N aqueous solution of sodiumhydroxide was added while stirring to adjust the pH to 10. Then, using amixer, the resulting product was refined to obtain a transparentcellulose nanofiber dispersion.

Production Example 2

Water was added to 5 g of a TEMPO-oxidized cellulose prepared in thesame manner as in (1) of Production Example 1 above to adjust the solidcontent to 1%, and a 1 N aqueous solution of sodium hydroxide was addedwhile stirring to adjust the pH to 10. Then, using a mixer, theresulting product was refined to obtain a transparent cellulosenanofiber dispersion.

Production Example 3

Water was added to 5 g of a TEMPO-oxidized cellulose prepared in thesame manner as in (1) of Production Example 1 above to adjust the solidcontent to 1%, and a 10% aqueous solution of tetraethylammoniumhydroxide was added while stirring to adjust the pH to 10. Then, using amixer, the resulting product was refined to obtain a transparentcellulose nanofiber dispersion.

Production Example 4

Water was added to softwood bleached kraft pulp (40 g by dry weight) toadjust the solid content to 2%. The mixture was roughly broken up by amixer. The mixture was then repeatedly treated with a grinding mill toobtain a white creamy cellulose nanofiber dispersion.

Comparative Example 1

The cellulose dispersion obtained in Production Example 1 and thecellulose nanofiber dispersion obtained in Production Example 4 weremixed at a ratio (weight ratio) of 8:2; the mixture was casted onto apolystyrene container and dried in an oven at 50° C. for 24 hours toobtain a cellulose nanofiber sheet having a thickness of 20 μm. Theevaluation results of the physical properties are shown in Table 1.

Comparative Example 2

A cellulose nanofiber sheet having a thickness of 20 μm was obtained inthe same manner as in Comparative Example 1 with the exception that thecellulose dispersion obtained in Production Example 1 and the cellulosenanofiber dispersion obtained in Production Example 4 were mixed at aratio of 5:5. The evaluation results of the physical properties areshown in Table 1.

Comparative Example 3

A cellulose nanofiber sheet having a thickness of 20 μm was obtained inthe same manner as in Comparative Example 1 with the exception that thecellulose dispersion obtained in Production Example 1 and the cellulosenanofiber dispersion obtained in Production Example 4 were mixed at aratio of 2:8. The evaluation results of the physical properties areshown in Table 1.

Comparative Example 4

A cellulose nanofiber sheet having a thickness of 20 μm was obtained inthe same manner as in Comparative Example 2 with the exception that thecellulose dispersion obtained in Production Example 2 was used insteadof the cellulose dispersion obtained in Production Example 1. Theevaluation results of the physical properties are shown in Table 1.

Comparative Example 5

A cellulose nanofiber sheet having a thickness of 20 μm was obtained inthe same manner as in Comparative Example 2 with the exception that thecellulose dispersion obtained in Production Example 3 was used insteadof the cellulose dispersion obtained in Production Example 1. Theevaluation results of the physical properties are shown in Table 1.

TABLE 1 Temperature Combined Light transmittance (%) at which sulfuricAverage Viscosity-average 450 nm weight loss acid degree of degree ofCrystal (100° C., Elongation reached 5% content substitutionpolymerization structure 660 nm after 3 h) (%) (° C.) (ppm) (—) (—)Example 1 Cellulose 85.5 84.3 8.0 284 290 2.35 585 triacetate I crystalComparative Cellulose I 90.8 90.2 3.5 — 0 0 — Example 1 crystalComparative Cellulose I 87.6 87.2 5.0 — 0 0 — Example 2 crystalComparative Cellulose I 76.3 74.1 6.5 — 0 0 — Example 3 crystalComparative Cellulose I 88.2 85.1 5.0 — 0 0 — Example 4 crystalComparative Cellulose I 88.5 85.4 6.8 — 0 0 — Example 5 crystal

Compared to the cellulose nanofiber sheets of the Comparative Examples,the film of the Example maintained comparable light transmittance whileexhibiting superior elongation. Therefore, it can be seen that the filmof the Example has both excellent bending properties and hightransparency.

The invention claimed is:
 1. A cellulose acetate film comprisingcellulose acetate having a cellulose triacetate I crystal structure, thecellulose acetate film having a light transmittance of 70% or higher at660 nm.
 2. The cellulose acetate film according to claim 1, having anelongation of 7% or higher when conditioned at 23° C. and 50% relativehumidity.
 3. The cellulose acetate film according to claim 1, wherein atemperature at which a weight loss of the cellulose acetate filmrelative to weight at 100° C. reaches 5% is 200° C. or higher when thecellulose acetate film is heated at a heating rate of 10° C./min under anitrogen atmosphere.
 4. The cellulose acetate film according to claim 3,wherein the temperature at which the weight loss of the celluloseacetate film reaches 5% is 250° C. or higher.
 5. The cellulose acetatefilm according to claim 1, wherein a combined sulfuric acid content inthe cellulose acetate is 20 ppm or greater and 500 ppm or less.
 6. Thecellulose acetate film according to claim 1, wherein the celluloseacetate has an average degree of substitution from 2.0 to 3.0.
 7. Thecellulose acetate film according to claim 1, wherein a diffractionprofile obtained from powder X-ray diffraction of the cellulose acetatefilm has peaks at two positions of 20=7.2 to 8.0° and 20=15.5 to 16.3°.8. A method for producing a cellulose acetate film, the methodcomprising: acetylating raw material cellulose by reacting the rawmaterial cellulose with acetic anhydride in a solvent containing a poorsolvent for cellulose acetate and acetic acid; diluting the celluloseacetate obtained by the acetylation with a dispersion medium to preparea dispersion; fibrillating the cellulose acetate in the dispersion;removing non-fibrillated fibers from the fibrillated cellulose acetate;dialyzing cellulose acetate from which the non-fibrillated fibers havebeen removed against water; and drying the dialyzed cellulose acetate toform a film.
 9. The method for producing the cellulose acetate filmaccording to claim 8, wherein the step of removing the non-fibrillatedfibers from the fibrillated cellulose acetate is performed bycentrifuging to separate into supernatant and precipitate, and removingthe precipitate.
 10. The method for producing the cellulose acetate filmaccording to claim 9, wherein the supernatant obtained by centrifugingis dialyzed against water using a dialysis membrane to form a dialyzedsupernatant.
 11. The method for producing the cellulose acetate filmaccording to claim 10, wherein cellulose acetate is formed into a filmby concentrating the dialyzed supernatant, placing the dialyzedsupernatant in a mold, and drying the dialyzed supernatant.
 12. Themethod for producing the cellulose acetate film according to claim 8,wherein the poor solvent for cellulose acetate is at least one selectedfrom the group consisting of benzene, toluene, xylene, cyclohexene,hexane, and amyl acetate.
 13. A cellulose acetate film consisting ofcellulose acetate having a cellulose triacetate I crystal structure, thecellulose acetate film having a light transmittance of 70% or higher at660 nm.